Answer:
I don't know if I am right or not. But I would say 702. He is planting a variety of tomatoes, no other crops are being planted except different types of tomatoes. Since they are all tomato seeds, and he is planting 702 tomato seeds.
I'm sorry but the solution you proposed was a bit hard to follow, so I'll just post my solution: we convert "2 and 1/2" to a single fraction:

So, we know that 5/2 gallons cost 60 dollars, which means

To find the cost of a single gallon, we have to solve the equation for g:

$3.50 when you step into the cab,
and then for however miles you drive add 2.25x
39/10
And/or
3 9/10
And/Or
3.9
The answer to your question is Y=5